In the manufacture of metal cans, it is conventional practice to form an unfinished can having a cylindrical configuration with a closed bottom portion and a cylindrical side wall that is open at the top end. Unfinished cans may be formed by a variety of processes including impact extrusion and deep drawing processes whereby a generally circular flat metal plate or sheet is first passed through an impact extrusion or deep drawing press where it is formed into a relatively shallow cup. The cup is then passed through an ironing press which elongates the side walls and produces the proper wall thickness of the can.
It is not possible, however, to produce a can having the exact height required or having the upper edge of the side wall exactly square. Can bodies as initially formed are usually of non-uniform height and may have irregular edges at the open top end. Subsequent to the formation of the unfinished container, it is necessary to trim the upper end of the cylinder to produce a can of a fixed height having a smooth upper edge.
It is essential that the upper edge of a can be smooth. The operation employed in many devices of the prior art of forming the flange along the upper edge of the can for attachment of a can lid often produces an uneven or cracked flange perimeter which, of course, can yield an inadequate seal between the lid and the upper edge of the can. Any imperfections such as burrs or cracks will prevent a proper seal between the lid and the can in the flange area. Thus, the forming of a flange having a smooth edge is essential to efficient can making.
The elimination of such imperfections is a particularly troublesome matter in the can making industry. One method of addressing the problems has been to form and trim the flange along the top of a can at the same time that the can is drawn or extruded. A significant drawback to this method is that scrap material from the trimmed can is often difficult to separate from the press and must be removed before the next run can be initiated.
An apparatus is needed to avoid this problem by efficiently trimming the can and discarding the scrap separate from the trimmed can. The irregular edge of the flange must be evenly trimmed without imperfections to produce identical mass-produced cans which can be efficiently sealed.